The goal of this proposal is to investigate the role of a putative liver-specific growth factor, hepatic stimulator substance (HSS), in liver regeneration and compare its effects with other mitogens, e.g., insulin like growth factors or transforming growth factors. Specific experiments will be performed as follows: 1) In vitro cell cycle specific effects of HSS and other mitogens in defined culture conditions will be characterized with HSS responsive, putative liver stem cells of epithelial origin (FNRL). Cell cycle curves will be generated by (3H)-thymidine incorporation and cell cycle subpopulations will be analyzed by flow cytometry. To determine which cell subpopulation is specifically responsive to HSS, GO/GI, S, or G2/M phase cells will be purified by centrifugal elutriation and exposed to mitogens. Simultaneous analysis will be performed of cell cycle related gene expression (e.g., c-fos, c-myc, c-Ha-ras). In vivo experiments in rats will determine whether HSS induces similar changes in gene expression. 2) To study whether HSS induces early events in membrane signal transduction, as opposed to later events such as nuclear gene activation, Na+ and Ca++ ion fluxes will be determined by NMR spectroscopy and tyrosine kinase activation will be determined by a peptide phosphorylation assay. 3) Whether HSS enhances in vivo proliferation of hepatocytes, or hepatocyte precursor cells with or without an introduced albumin gene, will be determined by transplanting HSS pretreated cells, or by administering HSS following transplantation into normal and analbuminemic rats.